The invention relates to methods of interfacial polymerization and to novel products made possible by such methods, such as membranes.
Membranes are used, for instance, in separation processes as selective barriers that allow certain chemical species to pass, i.e., the permeate, while retaining other chemical species, i.e., the retentate. Membranes are used in many applications, for example as inorganic semiconductors, biosensors, heparinized surfaces, facilitated transport membranes utilizing crown ethers and other carriers, targeted drug delivery systems including membrane-bound antigens, catalystcontaining membranes, treated surfaces, sharpened resolution chromatographic packing materials, narrow band optical absorbers, and in various water treatments which involve removal of a solute or contaminant for example dialysis, electrolysis, microfiltration, ultrafiltration and reverse osmosis.
There is a myriad of supports or substrates for membranes. Specific physical and chemical characteristics to be considered when selecting a substrate include: porosity, surface area, permeability, solvent resistance, hydrophilicity, flexibility and mechanical integrity. Other characteristics may be important in certain applications.
As the use of porous membranes increases, so does the need to find new ways to modify, or functionalize, the membrane or membrane substrate. Without modification, effectiveness of the membrane is restricted by the nature of the membrane or membrane substrate material itself. Conversely, modification or functionalization of the surfaces of the membrane substrate can increase the usefulness of the substrate and can open up new areas of application.
There are already known methods of functionalizing the internal pore surfaces of a porous substrate. One method is radiation grafting which has the advantage that a wide variety of monomers can be used which permit further modification. Disadvantages with this method are that the radiation may degrade the substrate itself and the method requires expensive equipment. Another method of functionalizing is the coating method which has the advantage of being a simple procedure and is available for a variety of polymers. However, the coating method often results in blocking of the pores of the substrate and generally it is necessary to post crosslink in order to anchor the polymer to the substrate, otherwise the finished membrane may be unsuitable for use with some solvents as the solvents may remove the coating of the membrane from the membrane substrate. Another method involves the use of pre-functional resin to make the substrate which is then converted into some desirable end product substrate. This is a simple method but functionality is wasted throughout the bulk of the membrane. Further, post crosslinking may be needed if the membrane is to be used with solvents. Yet another method is oxidative derivatization which is simple but is limited in the choice of substrate. Further, the substrate may be degraded and there is insufficient control of the functionality.
There is no single method of functionalizing internal pore surfaces that is ideal for every situation. There is always a need for additional methods to modify, functionalize or form active surfaces for membrane applications.
Interfacial polymerization has been used to prepare thin film composite membranes. Interfacial polymerization is a process in which a very thin film can be made by reacting two, or more, monomers at an interface between two immiscible phases. It is best described by example. "Nylons" belong to a class of polymer referred to as polyamides. One such polyamide is made, for example, by reacting a diacid chloride, such as adipoyl chloride, with a diamine, such as hexamethylene diamine. That reaction can be carried out in a solution to produce the polymer in resin form. Alternatively, the reaction can be carried out at an interface by dissolving the diamine in water and floating a hexane solution of the diacid chloride on top of the water phase. The diamine reacts with the diacid chloride at the interface between these two immiscible solvents, forming a polyamide film at the interface which is rather impermeable to the reactants. Thus, once the film forms, the reaction slows down drastically, so that the film remains very thin. In fact, if the film is removed from the interface by mechanical means, fresh film forms almost instantly at the interface, because the reactants are so highly reactive with one another.
The discovery of interfacial polymerization in the late fifties provoked interest from the textile industry. Natural fiber textile manufacturers, particularly wool manufacturers, were looking for ways to make their textiles shrinkproof. In early experiments wool swatches were soaked in a diamine, excess diamine was squeezed out by passing the fabric through nip rollers, and the fabric was soaked in an acid chloride polymerizable with the diamine.
Numerous condensation reactions that can be used to make polymers interfacially have been described. Among the products of these condensation reactions are polyamides, polyureas, polyurethanes, polysulfonamides and polyesters. Factors affecting the making of continuous, thin interfacial films include temperature, the nature of the solvents and cosolvents, and the concentration and the reactivity of the monomers. Refinements which have been developed include the use of `blocked` or protected monomers that can be later unblocked to alter the chemistry of the finished film or membrane, the use of post-treatment of the films to alter their chemistry, and the use of heteroatoms in the monomers to alter the properties of the final film or membrane. In the classical organic chemistry sense, these alterations or modifications can be referred to as changes in the functionality, i.e., in the available functional groups of the monomers and/or polymers, hence functionalization.
The use of interfacial polymerization to produce extremely thin film on a support is known. Such polymerization can be carried out by dissolving one monomer in a solvent and then using that solution to saturate the substrate. The outer surface of the substrate, saturated with the first solution, is then exposed to a second solution, immiscible with the first solution, containing a second monomer. A very thin film of polymer is formed at the interface of these two solutions on the outside surface of the substrate.
In the above process the substrate serves as a mechanical support for the thin film formed by interfacial polymerization. The thin film itself extends across and blocks any pores present in the substrate. Thus, this use of interfacial polymerization does not take advantage of the great surface area available within a porous substrate.